Analysis of geological samples by atomic emission spectroscopy of plasmas induced by laser ablation at low pressures

Kurzfassung

Elemental analysis of geologic samples based on atomic emission spectroscopy is currently considered for several future space lander missions to planets, moons and asteroids in solar system. The spectroscopy technique, called laser-induced plasma (breakdown) spectroscopy (LIBS), provides quantitatively the microscopic in-situ abundances of all major and many trace elements of surfaces of solar system bodies. Excitation and evolution of the plasmas induced by lasers depend on the properties of the investigated material and on environmental conditions. This study focuses on feasibility of spectroscopy of plasmas induced by low-energy laser (below 1 mJ per pulse) for exploration of solar system bodies with thin atmospheres (pressures below 1 mPa). At such low pressures the excited plasmas have small plumes and expand very rapidly, which limits both the LIBS signal intensity and the available acquisition time. This, in turn, requires usually relatively powerful laser sources to create a detectable LIBS plasma. The low pressure conditions were simulated in a dedicated chamber at DLR-Berlin, that can hold the Martian-like atmosphere or high vacuum conditions. Two infrared Q-switched lasers are used for comparative investigation of atomic emission spectra: Nd:YAG laser (Inlite, Continuum) operating at 1064 nm and at 10 Hz, pulse energy up to 230 mJ at 8-10 ns pulse duration and developed for future planetary missions Nd:YLF laser (NeoLASE) operating at 1053 nm and at 10-50 Hz, pulse energy up to 3 mJ at 3-5 ns pulse duration. The emitted light of the laser-induced plasma was analysed by an echelle spectrometer (LTB Aryelle Butterfly) covering the wavelength range of 280–900 nm with a spectral resolution of around 104. Identification of atomic transitions was performed using the LTB built-in spectrometer software by comparison with the NIST spectral database. Several basaltic rock and sediment standards were crushed to powder and pressed into pellets. Reduction of both pressure and a laser excitation energy results in significant decrease of signal-to-noise ratio for most of atomic lines (an exception are the widely broadened lines of H). However, detection of atomic emission lines of elements with relative abundances above 10-3 (0.1 wt%), in presented samples: Al, Ca, Cr, H, K, Mg, Mn, Na, Ni, O, Si, Ti,  was possible down to a laser excitation energy of ~0.9 mJ (provided laser irradiance on a sample surface of 46 MW/mm2). Additionally, detection of carbon and sulphur, having strong atomic transitions in ultraviolet range, can be expected by extension of the spectral range of a LIBS spectrometer to 190 nm. Atomic doublet and triplet transitions, broadened by atomic collisions at ambient pressures (100 kPa), become spectrally resolved and are identified below 1 mPa. This demonstrates the feasibility of miniaturized laser-induced breakdown spectrometry for space missions to solar bodies with absent or thin atmospheres.